Antibacterial Activities of New Schiff Bases and Intermediate Silyl Compounds Synthesized from 5-Substituted-1 , 10-phenanthroline-2 , 9-dialdehyde

Schiff bases are known to possess anticancer, antibacterial, antifungal, antitubercular, anti-inflammatory, antimicrobial and antimalarial properties. In this paper antibacterial studies against variety of plants and human pathogenic bacteria with eight newly synthesized Schiff bases and several intermediate silyl compounds have been reported. The antibacterial activities of the synthesized compounds were primarily determined by paper disc diffusion method. The minimum inhibitory concentration (MIC) of each compound was also determined by tube dilution process. Seven different human pathogenic bacteria and eighteen different plant pathogenic bacteria were used for the antibacterial activity studies. While all synthesized compounds have shown significant antibacterial activity, one intermediate silyl compound has shown remarkably high antibacterial property. 5-substituted derivatives have shown relatively higher activity than non-substituted compounds. Polar substituent which increases hydrophilicity may have a positive impact on the antibacterial property.


Introduction
Schiff base, named after Hugo Schiff, is another name of iminie functional groups or azomethine groups (-C=N) that are classically formed by condensation of a primary amine with an aldehyde or ketone [1].Since various Schiff bases and their complexes have been formed and exhibited promising activity against bacteria and other microorganism, biochemists are interested to study Schiff bases for their medical importance and use in design of medicinal compounds [2] [3].Many Schiff bases have antimicrobial and antifungal activities [4].Additionally, aryl groups or heterocyclic residues containing Schiff bases possess excellent biological activities [5] [6].
Phenanthroline and its derivatives have significantly antibacterial, antifungal and anticancer activity [7].Phenanthroline metal complex interacts with DNA by cleavage of the DNA strand involving interaction of the metal center with a phosphate group and cleavage of the phosphate ester bond [8] [9].Also, some phenanthroline Schiff bases and their complexes are found to be photocleavers of pUC19 DNA in visible light and cytotoxic in HeLa (human cervical cancer) and MCF-7 (human breast cancer) cells in visible light [10].Phenanthroline complexes exhibited high cytotoxicity in cancer cell lines, even sometimes higher activity than cis-platin and substitution at 5-or 5,6-position increases cytotoxicity such as the S, S isomer of complex is 100-fold more cytotoxic than of cis-platin in L2110 murine leukemia cell lines [11].
Schiff bases of thiosemicarbazone are very important due to their antimicrobial action [12].Also, Schiff bases and their metal complexes having amino thiol group (HN 2 -NH-CS-R) show various biological activities.Thiosemicarbazone containing compounds can be used as drug against tuberculosis, leprosy [13] and tumor [14].
The biological study of Schiff bases containing 5-substituted-1,10-phenanthroline-2,9-dicarboxaldehyde with amines containing thione (C=S) groups has not been studied precisely.We, therefore, report here the antibacterial study of eight new Schiff bases containing 5-subntituted-1,10-phenanthroline moiety with different thiosemicarbazones.The intermediate silyl and aldehyde compounds are also studied against these bacteria.

Preparation of test microorganism
The sterilized (autoclaved for 30 min) medium was poured into sterilized petri dish (500 ml for 20 petri dishes) and solidified (1.5%, w/v agar) at room temperature for 30 min.18 different types of plant pathogenic and 7 human pathogenic bacterial strains were used in this study of both gram positive and gram negative bacteria.A small amount of bacteria was collected from bacteria seed and streaked it onto an appropriate media plate for the bacteria.The plate was placed in a incubator for 24 h at 28˚C for plant pathogenic bacteria and at 37˚C for human pathogenic bacteria.

Preparation of Reagents
Fifteen compounds including eight new Schiff bases, four intermediates and one starting molecule were used for antibacterial screening test by using disk diffusion method (Table 1).The eight new Schiff bases were synthesized and characterized by using different spectroscopic methods including IR, 1 H NMR, 13 C NMR and Mass spectroscopy [15].The concentration of the test compounds was 20 mg/ml.The concentration for standard, tetracycline (TC), was 10 mg/ml for screening test.Test compounds were dissolved in dimethylsulphoxide (DMSO).The name of the compounds and their structure are given in Table 1.

Antibacterial Activity Screening Test (Disc Diffusion Method) [16]-[18]
2-3 bacteria colonies were taken in appropriate liquid media and shook it in an incubator shaker for 5 h at 28˚C for plant pathogenic bacteria and at 37˚C for human pathogenic bacteria.The spinning speed maintained at 180 ppm.The bacteria solution was then diluted four times and spread 0.15 ml of this fresh pure culture on media plate with a help of spreader under aseptic condition.Sterile 6 mm whatman filter paper was used which was impregnated with 10 µl of sterile stock solution (20 mg/ml) of the compound.The filter paper with solution was  placed in the middle of the bacteria spread media plate.The plates were incubated at appropriate temperature for 24 h.The diameter of the inhibition zone observed was measured in mm scale.Disc impregnated with sterile DMSO was used as a control and with sterile tetracycline (TC) was used as antibacterial reference standard.All tests were performed under sterile conditions in duplicate and repeated three times.

Determination of Minimum Inhibitory Concentration (MIC)
Minimum Inhibitory Concentration (MIC) indicates the lowest concentration of an antimicrobial agent that inhibits the visible growth of a microorganism after overnight incubation.To determine MIC, different variable concentrations of the compounds were used.Media plate and bacteria culture were prepared the same way that were done before in the disk diffusion method.After spreading about 0.15 ml diluted bacteria on the media plate, seven holes were punched on the plate for seven different solutions.25 µL of each solution (T c , T 0 , T 1 , T 2 , T 3 , T 4 , T 5 ) was put in each hole according to the concentration and kept it in incubator at appropriate temperature for 1 day.The diameter of the inhibition zone was measured in mm scale and recorded the data.All the tests were repeated three times.

Antibacterial Activity Screening Test
The antibacterial study is based upon a comparison of inhibition of growth of bacteria by test compounds with that produced by standard antibiotic, Tetracycline (TC).Inhibition zone is the clear region around the paper disc saturated with an antimicrobial agent.The screening result showed that all the compounds exhibited noticeable antibacterial activity against different bacteria.The following Tables show the name of the compounds and antibacterial activity according to the inhibition zones in mm scale.
The antibacterial activity of compounds against 7 human pathogenic bacteria is presented in Table 2.The bar charts represent a comparison of the activities of 5-substituted Schiff bases with similar Schiff bases without the substituents.All compounds found to possess antibacterial activity against all human pathogenic bacteria.In fact, compounds showed higher activity for human pathogenic bacteria than plant pathogenic bacteria.Also, it is observed very high activity for silyl derivative, N (Figure 1 and Figure 4).The silyl derivative showed higher activity than tetracycline (TC) in case of Staphylococcus aures, Streptococcus pyogenes, Bacillus subtilus and Listeria monocytogenes. Figure 2 and Figure 3 show graphical representations of antibacterial activity against human pathogenic bacteria.These bacterias are E. Coli MC4100, E. Coli DH5a, Staphylococcus aures, Streptococcus pyogenes, Yersinia enterocoitica 27729, Listeria monocytogenes and Bacillus subtilus.
Nine different bacteria of Pseudomonas genre were tested.The result is presented in Table 4.In this case, nitro and bromo aldehyde (A and F) showed higher activity against Pseudomonus Cichorii 302959, Pseudomonas syringae 728α and Ps.Pv.Morspruorum 218-01 (Figures 7-9).

Conclusion
Antibacterial studies of 15 different compounds including intermediate silyl and aldehyde compounds were carried out.These compounds were tested on 18 plants and 7 human pathogenic bacteria.The antibacterial activity screening test of the synthesized compounds was initially examined by paper disc diffusion technique.In screening test, the compounds showed higher antibacterial activity for human pathogenic bacteria.High activity was observed for silyl derivative as well.Compound N was even more active than the standard tetracycline (TC).Moreover, it has been found that the activity of the 5-substituted Schiff bases has shown higher activity than their non-substituted members.Finally, the minimum inhibitory concentrations (MIC) of the compounds were determined by tube dilution method.For most of the compounds MIC was 100 μg/ml.

Figure 1 .
Figure 1.Photos of the antibacterial test of compound N and standard.

Figure 2 .
Figure 2. Antibacterial activity test against four human pathogenic bacteria.

Figure 3 .
Figure 3. Antibacterial activity test against three human pathogenic bacteria.

Figure 5 .
Figure 5. Antibacterial activity test against four plant pathogenic bacteria.

Figure 6 .
Figure 6.Antibacterial activity test against five plant pathogenic bacteria.

Figure 7 .
Figure 7. Photo of antibacterial test of bromo, F and nitro aldehyde, A against Pseudomonas cichorii.

Figure 8 .
Figure 8. Antibacterial activity test against four plant pathogenic bacteria.

Figure 9 .
Figure 9. Antibacterial activity test against five plant pathogenic bacteria.

Figure 10 .
Figure 10.Pictures of minimum inhibitory concentration (MIC) test for the Compound N.

Figure 11 .
Figure 11.MIC of Silyl compound, N against different bacteria.

Table 1 .
Names of the compounds that are used in antibacterial study.

Table 2 .
Antibacterial activity of the compounds against human pathogenic bacteria.

Table 3 .
Antibacterial activity of nine plant pathogenic bacteria.

Table 4 .
Antibacterial screening test of nine bacteria of Pseudomonas genre.

Table 8 .
Overall summary of MIC.